YouTube wants to show you bits and pieces of a video while you’re browsing on the web. The streaming site started testing out video previews a couple of weeks back but they’re now rolling it out to more users. If you hover your pointer over a video, you’ll start to see a three-second teaser of the clip. It supposedly works for videos that are longer than 30 seconds. Not all creators are getting this preview though. According to YouTube, it’ll depend on “topic and content.” You’ll see the previews on the Home page, Search results page, Watch page, Subscriptions tab, and Trending tab on Chrome and Opera.
Source: YouTube Help Forum